But the heroes themselves can't break the dome, right? It's effectively unbreakable? So why worry about Chituari so much? If these powerful heroes can't sent it, the less powerful aliens that the heroes can defeat soundly may be able to?

First, the heroes may be more powerful than any single wave of Chitauri, but they are not more powerful than all the Chitauri put together. In any case, if the Chitauri get through the shield, then the heroes won't be able to prevent the immense collateral damage the Chitauri would be able to inflict. The heroes can't know with absolute certainty that just because they can't get through it means the Chitauri can't get through hit. Finally, if the heroes do down the shield or Hydra Cap decides to down it on his own, then the Chitauri are free to wreak havoc all over Earth so the heroes have to be ready for that. Spoiler: The heroes do get through the shield in the most recent issue of Secret Empire.